1. Field of the Invention
This invention relates to a passive optical ring network, and in particular to a passive optical ring network for use in an in-station telecommunications environment, or for use in a local area network (LAN), a wide area network (WAN), or a metropolitan area network (MAN).
2. Related Art
In a typical in-station environment, such as a trunk repeater station, telecommunications signals are fed into tributary cards at 2 Mb/s from local exchanges. These signals are then passed to an 8 Mb/s muldex (multiplexer/demultiplexer), from where they pass to a 140 Mb/s muldex via a 34 Mb/s muldex. At each stage, eight interconnections and 16 point-to-point co-axial links are required, and this gives rise to termination problems on the associated cards.
In order to reduce the number of interconnections and point-to-point links, it is known to use a passive optical ring of the type shown in FIG. 1, this figure showing how such a ring can provide the necessary interconnections between four tributary cards 1 and an 8 Mb/s muldex 2. Thus, each of the cards 1 and the muldex 2 is connected to a passive optical ring 3 by a respective passive 50/50 optical coupler 4. In this way, the cards 1 are connected to the muldex 2 by a ring (the passive optical ring 3) having only five nodes (the couplers 4), and this leads to a substantial reduction in the number of terminations on the card associated with the muldex 2.
As a trunk repeater of this type only carries traffic at 2 Mb/s and 140 Mb/s (signals come in at 2 Mb/s from the local exchanges, and go out over the trunk network at 140 Mb/s), the 8 and 34 Mb/s muldexes generally carry no traffic, and so are surplus to requirements from the point of view of traffic management. In modern trunk repeater stations, therefore, it would be advantageous to feed 2 Mb/s tributary cards directly to the 140 Mb/s muldex.
With standard co-axial interconnections, this would cause severe termination problems, as 128 point-to-point links would be required to serve 64 2 Mb/s tributaries. Moreover, the passive ring topology of the type described above with reference to FIG. 1 would not work, as such a ring could not support the required number of nodes. Thus, assuming edge emitting light Emitting diodes (ELEDs) are used to launch power into the couplers 4, and the couplers are associated with receivers of sensitivity -52 dB, the optical power budget (that is to say the difference between receiver sensitivity and launch power into a fibre at a coupler) of the ring of FIG. 1 can support only five nodes. This is because the launch power of the ELEDs is -26 dB, (giving an optical power budget of 26 dB), there is a loss of -4 dB at each coupler 4, and typically there are fibre and connection losses of -2.5 dB. Clearly, therefore, this ring topology cannot cope with the 64 node terminations required.
It would be possible to increase the number of nodes such a ring would support up to 15, by using lasers instead of ELEDs, and by using receivers of better sensitivity. The disadvantage of such an arrangement would be its high cost; and, even then, it could not provide the required 64 node terminations. A further increase to 33 nodes would be possible if the 50/50 couplers were replaced by 90/10 couplers, and the network configured to give minimum path loss between all the tributaries and the muldex, but this would have the additional disadvantage of dictating which node is a transmitter and which is a receiver. Consequently, this arrangement could not be used for duplex operation, and so is unlikely to meet all the future needs of the in-station environment. Moreover, the required target of 64 node terminations would still not be met. If the 90/10 couplers were used symmetrically, however, only 26 node terminations would be possible, due to an 11 dB loss which would occur at each coupler because both transmission and receive nodes would be present.